Add support for custom e-mail formatting via Jinja templates

[jwodder]

The config file should gain a subtable in which the user can specify Jinja templates for rendering each event type and the e-mail as a whole.

• The event details should be present in an event template's context as an event: reponews.types.Event variable.
• The template for the e-mail as a whole should be passed the events as an events: List[reponews.types.Event] variable, and each individual event is templated by calling a special render filter on it (not to be confused with Jinja's render() method; rethink name).
• Strip trailing newlines from each template after rendering?
• The templating subtable should have an environment_options (Rethink name) subtable giving parameters that will be passed directly to jinja2.Environment.
• The templating subtable should have an html = BOOL key (default false) that, when set, causes reponews to use HTML instead of plain text templates as the default templates and also causes the final e-mail to be declared with Content-Type: text/html.
• As in Add an option for dumping events as JSON #3, the "author" fields of events should be renamed to "user" first for consistency.
• For consistency with other activity classes, NewForkEvent should gain a user property that just returns .fork.owner.
• The templating subtable should include keys for controlling where included/imported templates should be loaded from.